1. Field of the Invention
This invention relates to a thermal barrier valve and more specifically to a cover and insulator for a thermal barrier poppet valve used in a combustion chamber of a piston engine.
2. Description of the Prior Art
Covers or thermal barriers for protecting poppet valves in piston engine combustion chambers are well-known. For many years such covers or barriers have been proposed for the purpose of reducing heat corrosion and preventing weakening of the valve head, to improve or enhance combustion, to reduce heat conduction through the valve head, etc. These and other purposes are provided by the poppet valve thermal barriers disclosed in the above mentioned applications. Therein, the thermal barriers include a one-piece, cup-shaped, sheet metal cap or cover having a shield portion spaced from the valve face and integrally formed with a cylindrical skirt portion welded to the valve head to define an evacuated chamber. The evacuated space in the chamber is an excellent conductive heat flow barrier. The chamber is preferably filled with an insulating material to reduce radiation heat flow across the evacuated chamber and to support the shield portion. The skirt portion, which is conductively connected to the relatively hot shield portion and to the relatively cool valve head, has a temperature gradient over its length between the shield portion and valve head or weld. This gradient causes the skirt to expand in a bellmouth fashion and thereby relieve or reduce destructive stresses in the shield and skirt due to thermal expansion of the shield.
Manufacture of the above thermal barrier can be rather time-consuming to ensure a high vacuum in the chamber. Further, since the skirt portion is conductively connected to the shield portion and the valve head, heat flow to the valve head via the skirt portion can circumvent much of the heat barrier properties of the evacuated chamber unless the metal thickness of the skirt is small.